Subterranean “sliding” drilling operation typically involves rotating a drill bit on a downhole motor at the remote end of a drill pipe string during a drilling operation. Drilling fluid forced through the drill pipe and downhole motor rotates the drill bit. The assembly is directed or “steered” from a vertical drill path in any number of directions, allowing the operator to guide the wellbore to desired underground locations. For example, to recover an underground hydrocarbon deposit, the operator may drill a vertical well to a point above the reservoir and then steer the wellbore to drill a deflected or “directional” well that penetrates the deposit. The well may pass horizontally through the deposit. Friction between the drill string and the bore generally increases as a function of the horizontal component of the bore, and slows drilling by reducing the force that pushes the bit into new formations.
Such directional drilling requires accurate orientation of a bent segment of the downhole motor that drives the bit. Rotating the drill string and adjusting the angular position of the pipe at the surface using a top drive or rotary table change the orientation of the bent segment and toolface. To effectively steer the assembly, the operator must first determine the current toolface orientation, such as via measurement-while-drilling (MWD) apparatus. Thereafter, if the drilling direction needs adjustment, the operator must rotate the drill string to change the toolface orientation.
Directional drilling is often performed at the end of a drill string that is several thousand feet in length. Although change of the borehole direction is typically accomplished through a gradual deflection over hundreds of feet or more so that the drill string bends gradually, the friction between the drill string and the borehole generally increases. In addition, the drill string is elastic and stores torsional tension like a spring. Consequently, when an operator makes a static angle adjustment to the drill string at the drilling rig to change the toolface orientation, a substantial portion of the angle adjustment is “absorbed” by the friction without changing the toolface orientation. Thus, the drill string can require more rotation at the surface than the desired rotation of the toolface.
Typically, mechanized drilling systems are controlled by setting limits to either the number of wraps or applied torque by estimating the amount of pipe twist and reactive torque of the bottom hole assembly (BHA). This estimate is used to set the angular position of the pipe at the surface until information from the MWD tool is received on the surface and then additional corrections are made based on the actual bend orientation measured by the MWD tool.
These methods can be slow due to transmission speed of receiving the appropriate downhole data at the surface. When this process is controlled manually by a directional driller, the driller is able to watch the pump pressure and/or weight on bit (WOB) at the surface and determine what the reactive torque of the downhole system will be before bend orientation from downhole is received. Reorienting the toolface in a bore can be very complex, labor intensive, and often inaccurate, and thus, automated methods and systems are desired.